Method of measuring moment of inertia

ABSTRACT

The method for measuring torque described is based upon the fact that angular velocity omega and angular acceleration Alpha are independent dynamical variables at any given time. Thus, for any given omega , a simultaneous determination of Alpha and omega can be utilized for a determination of power. The method of measurement requires a negligible perturbation of the rotating system. It is easily susceptible to modification over a wide range of variations.

United States Patent Ludloff Sept. 16, 1975 [541 METHOD OF MEASURING MOlV[ENT OF 3,505,863 4/1970 Lucia ..1 73/1 16 INERTIA 3,581,561 6/1971 Tomashek 3,657,922 4/1972 Sibeud 73/1 17.3 [76] Inventor: Karl Ludloff, 159 Midvale Ave,

Los Angeles Calif 90045 Primary Examiner.1erry W1 Myracle [22] Filed: May 14, 1973 Appl. No.: 359,851

Related U.S. Application Data CIRCUIT SUBSYSTEM 5 CIRCUIT CONTROL ELECTRON/C5 5 7 1 ABSTRACT The method for measuring torque described is based upon the fact that angular velocity u) and angular acceleration a are independent dynamical variables at any given time. Thus, for any given no, a simultaneous determination of a and m can be utilized for a determination of power. The method of measurement requires a negligible perturbation of the rotating system. It is easily susceptible to modification over a wide range of variations.

4 Claims, 3 Drawing Figures SCALING CIIZCUIT INDICATDR CIRCUIT METHOD OF MEASURING MOMENT OF INERTIA This is a division of application Ser. No. 132,007, filed Apr. 7, 1971.

BACKGROUND OF INVENTION The present invention is directed to a new method of measuring torque, based upon the fact that part of the torque produced by a system with rotating elements (hereinafter referred to as rotating systems) must be used to overcome the inertia of said rotating elements.

While we will use in this specification reference to measurements of power P, or of torque N, or of angular acceleration a, it should be understood that because of the relationships existing between these quantities, said relationships involving angular velocity to and moment of inertia and being known in this art, power, or torque, or angular acceleration may be used in the descriptive sense to include all three of these quantities as well as the time rate of change of the square of the angular velocity.

The difference in principle between the method for measuring power described herein and existing dynamometer measurements can be seen readily from the equation that relates the torque N on a system to the time rate of change of the angular momentum (dL/dt) of that system, i.e.

a'L/dt N. 1 This equation is the analogue, for a rotating system, of Newtons second law of dynamics, and is in fact derived from this law.

Existing dynamometers determine the torque produced by a contrivance by balancing it against a known external load, i.e., they determine the physical quantity represented by the right-hand side of equation given above. The method specified herein involves determining the time rate of change of the angular momentum of the rotating elements internal to the system, i.e., determining the physical quantity represented by the lefthand side of equation given above. This is done by making use of the relation between time rate of change of angular momentum, moment of inertia I, and the angular acceleration a (i.e., the time rate of change of angular velocity), resulting in the equation dL/dr la! 2 The moment of inertia is taken to be known, since this is a quantity which is either previously provided for a given system, or can be determined by those working in this art. The determination of a [angular acceleration] is combined with the known value of I [inertia] to give dL/dt and hence by equation 1, given above, the value of N to be determined.

The fundamentally new concept in the method specified herein is based upon the fact that angular velocity (.0 and angular acceleration a are independent dynamical variables at any given time. Hence, for any given w, a simultaneous determination of a and m can be utilized for a determination of power. Furthermore, the method of measurement specified herein entails a negligible perturbation of the rotating system (weak-probe measurement). In contrast, although present dynamometers allow determination of w by a weak-probe measurement, they require a strong coupling to an external system to determine torque (strong-probe measurement). For example, a tachometer measurement can presently be made during a vehicles normal performance without affecting the value of to being determined (weak-probe measurement). However, present methods dissipate practically the entire torque being measured (strong-probe measurement) and require taking the vehicle out of its normal mode of operation. The weak-probe method of determining torque specified herein thus leads to the first completely weakprobe dynamometer, allowing power measurements under fully dynamic conditions, and thereby opening whole new fields of application precluded with a strong-probe dynamometer, such as monitoring and control systems linked to continuous power determination in any desired mode of a vehicles operation.

It is contemplated that various instruments will accomplish the above measurements. It is further contemplated that a single device with suitable electronic circuitry will determine both of the independent variables w and a and thereby power, although such a device is not necessary for the method disclosed herein.

While throughout this description examples will be cited of devices, circuits and techniques, it should be understood that these cited examples are not to be taken in a restrictive sense, but are limited in scope and in number to improve the clarity and coherence of this disclosure. The proliferation of devices, circuits, techniques, and of applications of the method and devices will be readily apparent to those skilled in the art, and they are intended to be included as a part hereof.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a torquemeter and dynamometer circuit for single point measurements.

FIG. 2 is a block diagram of a torquemeter and a dynamometer circuit using some of the elements of FIG. 1, showing the method of eliminating the effect of slowly varying or fixed unknown loads.

FIG. 3 is a block diagram for a continuous reading dynamometer.

The torquemeter illustrated in FIG. 1 includes a contrivance (a rotating system) 10 whose output torque is to be determined. For example, if the contrivance were an automobile with a rotary engine, said output could refer to the direct torque output of the engine or to the torque output at the rear wheels, said latter torque differing from said former torque by system losses.

A tachometer 12 is coupled directly by conventional means to a rotating element of said contrivance 10, and is to measure the angular velocity w of said rotating element, a known relationship existing between the angular velocity of said rotating element and the angular velocity w of any other rotating element of said contrivance 10 which could be used to determine the output torque of said contrivance 10.

Different methods of measuring (0 and a are possible, such as electrical, electronic, mechanical, electromechanical, optical, and even nuclear methods. Furthermore while the terms electrical, electronic or electromechanical in describing a signal, circuit, device or measurement may be referred to herein, it should be understood that such terms are used in the descriptive sense to include any or all such terms when applicable.

The output 14 of said tachometer 12 is coupled by conventional means into an electronic circuit 16 that converts said output 14 into an electrical signal 18 called V representing the angular velocity to. The type of said electronic circuit 16 required, as described below, will depend in part upon said tachometer 12 used and the accuracy of measurement required.

Many suitable tachometers are well known to those skilled in this art and include, for example, (l) those based upon the'Faraday inductioneffect, and (2) those from: which binary outputs are obtained at fixed and known angular positions'ofa rotating element, as for example, from the breaker points of; an automobile ignition system. Y Y I I if said tachometer 12- is derived from the breaker points of a rotating engine,1then said electronic circuit 16 could, by conventional means, convert the rate of breaker-point closures into an electric analog signal, said analog signal being proportionalto the rate of breaker-point closures per unit time and hence proportional to the angular velocity of said breaker-point system. The lack of information concerning the state of the rotating system between breaker-point closures (usually referred to as dead time or resolution) limits the accuracy of measurements based on breaker-point systems, In actual practice dead time need not be a real limitation since the state of the art is such that angular position, or degrees, can be subdivided almost without limit down to microscopic levels, if necessary. Hence the time interval between successive binary outputs from a sufficiently sophisticated tachometer coupled to even the slowest rotating element of practical interest can be made vanishingly small, e.g., instantaneous.

An electronic circuit 20, whose input is the electrical signal 18, V w performs the mathematical operation of differentiation to yield as output 22 arrelectrical signal V a equal to the time rate of change of 18, V and representing the angular acceleration. Various known techniques are available to perform the desired operation, and a few electrical ones are given herein as illustrations,

l. Analog methodz' Utilizing conventional techniques, signal 18, V m is in effect applied across the terminals of a capacitor and a measurement is made of the current flowing through said capacitor as a result of the applied voltage 18, V w said current being directly related to the time rate of change of w and hence in effect, is a measure of output 22, V a

II. Digital method: V (t) and V (l-HDt) are measured, where Dt is a known time interval'and is smaller than any time interval of practical interest but large enough to achieve simplicity of electronic design. These two signals and Dtare combined utilizing conventional digital techniques to compute the desired result,

III. Instantaneous method: It is to be distinguished at this point in the description between determination of the instantaneous and the average angular acceleration a, exemplified by the analog and digital methods just referred to. The analog method is instantaneous since a continuous relationship exists between V w represent ing w, and V a representing a. The digital method, on the other hand, requires two successive measurements for the determination of V a and hence is not instan' taneous, although in the limit of a vanishingly small Dt it becomes instantaneous. We note further that inthe analog method, as applied to a breakerpoint type of tachometer, for example, althoughtthe determination of V a is instantaneous, that of V w becomes instantaneous only in the limiting case of a vanishingly small dead time. I

A selected value 24 of. w, called w is converted by electronic circuit 26 into the electrical signal 28, V

This can be accomplished by conventional means, for example, by a potentiometer mounted on the dashboard of an automobile or coupled to the throttle of an automobile or of -a boat. The electronic circuit 30 com pares the two signals 18, V and 28, V and, if the circuit 30 is in the enabled or ready-to-function state, it gives as an output the signal 32, G at the time when 18, V w passes through the value of 28, V The circuit 30 includes means for being enabled and .can include means for being disabled automatically after the signal 32, G or on-command from some other source. Signal 32, G also indicates whether 18, V m is increasing or decreasing when it passes through the value of 28, V although this information can be incorporated into the enabling circuitry. The techniques for realizing these functions are all well known in this art. On command from signal 32, G, the electronic circuit 34, by conventional means, causes thevalue of 22, V at that time to be stored. This particular valueof 22, V is called 36, V 0 and represents or at the time when 0) equals w The electronic .circuit 38 functions to account for the known moment of inertia 40, I of the system by providing for the multiplication of said signal 36, V 0 by said input], 40, the result ofsaid multiplication becoming the output signal 42from circuit 38, called N, and representing the uncorrected value of torque measured at to equal to co While the positioning of electronic circuit 38 is placed after circuit 34 as shown, it should be understood that said circuit 38 could have equally well, or even more simply, been positioned elsewhere, as will be readily apparent. The optional electronic circuit 44 modifies the signal 42, N to compensate for external load losses and for internal system losses, said losses called I, 46, in a manner discussed in more detail in connection with FIG. 2, said modified signal 48, N representing the corrected value of torque to be determined. The indicator 50 functions to display or record said value of torque N 48 to be determined. This indi cator 50 could consist, for example, of a simple panel meter, a recording device or a digital readout device and is optional. I

Electronic control circuitry 52 with interconnecting signal control paths 54, asrequired, is utilized to reset circuits to a prescribed state, to enable circuits as necessary, as for example the comparator circuit 30 or a tape readout device, to reset and disable appropriate circuits when their respective functions have been fulfilled, and to perform other. such conventional functions. I

The subsystem of elements 10 through 22, 28, 30, 32 and 54 in FIG. 1, which are described above, form a subsystem which will hereinafter be referred to as Subsystem S, 56, and indicated generally by the broken lines.

The torquemeter described above is combined with the electronic circuit 58 to form the dynamometer illustrated in FIG. 1. The circuit 58 mixes the signal 28,

i .V, with the signal 48, N to yield the output signal P,

for example a motor in motion, load conditions will vary, necessitating additional measurements. Next is considered how the effect of slowly varying or fixed unknown loads can .be eliminated to yield. thedesired; measurements, as is the case with. the? example .of torquemeter and .dynamometer illustratedin FIG- 2.. By slowly varying is meant sufficiently slowly varying to be considered constant over-the dutataion'ofthe measure-' ment.

In practice,--different models,varying-ampngs.t other things intheir'electronics, will berequired for different applications. It is convenient to distinguishin'particular two cases, one where-the rotatingsystemto be tested is housed ina stationary situation,and one wherethe rotating system is, for example, housed in a moving vehicle. The latter example also illustrates the-case where the load varies, dueto varying road conditions. The method is-illustratedby considering how the effectf the varying and unknown load can be eliminated to yield thedesiredpower measuremenn, I

Introduce the following notation:

N torque produced by the engine N B =torque associated with internal friction N, torque associated with external physical'q uantities,such as gravity, =road friction, etc.

1,, moment of inertia of engine systems l,, moment of inertia of rear-wheel'systems w,, angular velocity of engine w,, angular velocity of rear-wheel system rear-wheel drive.-) a I The quantity N will tend to diminish theuseful effect of N N, can act either to diminish it, e.g., by road friction and gravity acting on a vehicle movinguphill, or to enhance it,-e.g., by gravity acting on, a vehicle -moving downhill. The sign convention chosen for these quantities is apparent from equation 5. v

This then results in the basic equation setforth at the (assuming commencement of the description:

rate change of angular torque" torque momentum of engine and produced associated rear-wheel systems by engine with internal 'friction or, by the equation. v

V u u b A. NB NC (5) Since we consider a vehicle in gear, thejangularvelocities of engine and rear-wheel systems and theii time rates of change are related to each other, a relation which we express by the equations n X here K is a known number, whichhowever depends on the gear the vehicle is in and the makeand model of-the vehicle.-.

Using equation 6 we cannow eliminateone of two related quantities; say a from equation to give u h-) h A B (7 Under. steady-state conditions, i.e., when the yehicle is not accelerating, we have 01,, O, and henceN N N i.e.,

two sets ofmeas urementswill need to be made as follows:

I MEASUREMENT 2: Fuel Consumption at idling This measurement ismade under operating conditions by taking the foot off the accelerator while leaving the vehicle in gear. Under these conditions the torque produced by the engine is known and approximately equal to zero, V

Wehave from equation 7; Y

Combining equations 8 and 9 the frictional torques are eliminated to'obtain:

in terms of the. known torque produced while idling, the known parameters (K, I I,,) of the vehicle under the given operating conditions, and the two measured time rates, of change of the angular velocities. This torque measurementcombined with the measurement of the angular velocity .will give the required power.

It has beentacitly assumed that the load and the internal friction do not change specifically between the two measurements.

- Referring-now more particularly to FIG. 2, to make Ithese measurements, theselected value (0 24 is converted intoelectrical signal V 28, as before and fed into Subsystem S, 56. As expanded sample-and-hold circuit 34a new stores the value of a corresponding to torque produced torque associated with torque associated with by engine internal friction external quantities To measure by this method the torque produced-by the engine, the measurement is made? hilethe vehicle "is accelerating (or decelerating), how ver "slightly.\ To eliminate the effect of the varying and unknown load,

comparator circuit 30 in a different manner and which must allow for the storage control for two storage elements. The new electronic circuit 38a'must now take the difference between signals 36a and 36b before compensating for the known value of I, 40, for the rotating system. This is accomplished by'conventional techniques. In addition, although input I, 46 is no longer required, an optional input 62, N allows for the correction to the torque for the torque contribution when the system is in the quiescent state (for example, idling) should such a correction be necessary. The corrected value of torque to be determined is now called N 48a. Circuit 58 combines signals 28 and 48a to yield the,value of power 60 to be determined.

A further example of the method of this invention is illustrated in FIG. 3 showing an example of a continuous reading dynamometer. A device of this method has wide application in many types of monitoring and control systems where a continuous determination of power is required. For simplicity, there is considered the situation where the losses can be constant or slowly varying as described in connection with FIG. 1, although this is not a necessary requirement of a continuous reading dynamometer.

The dynamometer of FIG. 1 can be easily converted to a continuous'reading dynamometer, shown in FIG. 3. The subsystem of elements through 22 in FIG. 1, which are described above, form a subsystem which will herein be referred to as Subsystem S ,-56a. Its outputs are 22, V and 18,- V The circuit 58 mixes the signal 18, V with the signal 22, V to yield the output signal 64, V V equal the product of 18 and 22. Electronic circuit 38 functions to account for the known moment of inertia 40, I of the system to yield an uncorrected power 66, P Electronic circuit 44a functions to compensate for system losses 46, l to yield the corrected value of power 60, P to be determined. Indicator circuit 50 functions to display or record said value of P, 60 to be determined. Again, an optional electronic circuit 68 can be utilized to perform any necessary conventional tasks, such as resetting a circuit, via control lines 70.

Such a continuous dynamometer would, for example,

play a crucial, perhaps decisive, role in under-the-hood monitoring and control systems for automobiles, whose adoption appears to be forced by the kind of stringent pollution-control and safety requirements that have already been enacted in part. To ensure less exhaust pollution, for example, involves more 'efficient operation of the motors, which in turn requires monitoring a number of performance characteristics, amongst which power is one of the most crucial, during the vehicles normal operating conditions. A weak-probe compact dynamometer, such as the one described herein, is'essential for this purpose. The control system would then correlate quantities such as engine temperature, engine speed and power to regulate other performance characteristics, such as the fuel-injection mixture and igni- 1. A method for determining the moment of inertia'of a rotating system without the necessity of appreciable perturbation of the operation of said system, wherein the inertial properties of said system and the losses and using said measurement of angular acceleration to reload actingibm said system are constant or slowly'varylate said moment of inertia to said torque, changing said moment of inertia either by adding to it or by'subtracting from it a knownmoment of inertia, obtaining a second measure of angular acceleration of a rotating "element of said s'ystem under equivalent conditions with only said moment of inertia changed by said known amount, using said second measureof angular acceleration to relate said changed moment of inertia to said torque, establishing said moment of inertia tobe determined in terms of said known change in moment of inertia and said two values of angular acceleration by taking the difference between said two relations, averaging as many measurements as may be required to determine said moment ofinertia to specified accuracy, and making said determined moment of inertia available for further use. I 2. A method for determining the momentof inertia of a rotating system withoutthe necessity of appreciable perturbation of the operation of'said system, wherein the inertial properties of said system and the losses and load acting-on said system are constant or slowly vary ing, said method comprising; measurment at a particular angular velocity of angular acceleration of a rotating element of said system with a certain torque acting on said system, using said measurement of angular acceleration to relate said'momentof inertia to said torque, changing said torque either by adding to -it or by subtracting from it a known torque, obtaining a second measurement of angular acceleration of a rotating element of said system under equivalent conditions with only said torque changed by said known amount, using said second measurment torelate said moment of inertia to said changed torque, establishing said moment of inertia to be determined in terms of said known'torque and said two values of angular acceleration by taking the difference between said two relations, 3 averaging as many measurements as may be required to determine said moment of inertia to specified accuracy, and making said determined moment of inertia available for further use. 3. A method for determining the moment of inertia of a rotating system at a specified value of measurable system'parameters without the necessity of appreciable perturbation of the operation of said system, wherein the functional dependence of the torque acting on said system is known in terms of said system parameters, said method comprising; a simultaneous measurement by a controlled means at said specified value of said system parameters and with a certain non-zero torque acting on said system of angular acceleration of a rotating element of said system and of the particular value of said parameters, using said simultaneous measurement to relate angular acceleration to said moment of inertia and to said torqueat said particular value of said parameters, averaging as many measurements as may be required to obtain said relation to specified accuracy at said specified value of said parameters,

changing said moment of inertia either by adding to it or by subtracting from it a known moment of inertia,

obtaining a second simultaneous measurement of angular acceleration of a rotating element of said system and of the particular value of said parameters under equivalent conditions with only said moment of inertia changed by said known amount,

using said second measurement to relate angular acceleration to said changed moment of inertia and to said torque at said particular value of said parameters,

averaging as many said second measurements as may be required to obtain second said relation to specified accuracy at said specified value of said parameters,

establishing said moment of inertia to be determined in terms of said known change in moment of inertia and said two values of angular acceleration by taking the difference between said two relations at said specified value of system parameters, and making the result available for further use. 4. A method for determining the moment of inertia of a rotating system at a specified value of measurable system parameters without the necessity of appreciable perturbation of the operation of said system, wherein the functional dependence of the torque acting on said system is known in terms of said system parameters,

said method comprising;

a simultaneous measurement by a controlled means at said specified value of said system parameters and with a certain torque acting on the system of angular acceleration of a rotating element of said system and of the particular value of said parameters,

using said simultaneous measurement to relate angular acceleration to said moment of inertia and to said torque at said particular value of said parameters,

averaging as many measurements as may be required to obtain said relation to specified accuracy at said specified value of said parameters,-

changing said torque either by adding to it or by subtracting from it a known torque,

obtaining a second simultaneous measurement of angular acceleration of a rotating element of said system and of the particular value of said parameters under equivalent conditions with only said torque changed by said known amount,

using said second measurement to relate angular acceleration to said moment of inertia and to said changed torque at said particular value of said parameters,

averaging as many said second measurements as may be required to obtain second said relation to specified accuracy at said specified value of said parameters,

establishing said moment of inertia to be determined in terms of said known torque and said two values of angular acceleration by taking the difference between said two relations at said specified value of system parameters,

and making the result available for further use.

UNITED STATES PATENT OFFICE CERTlFICATE OF CORRECTION patent 3,905 ,223 Dated September 16 1975 Karl Ludloff Inventor (s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 46 after the equation cancel the exclamation mark. Column 7, line 32 after "equal" insert to There should be no space before the subscripts in the equations in the following places: Column 2 line 65 column 3, lines 28, 2 times line 30, 36 39, 40 41, 2 times line 42 3 times line 50 56 57, 59, 2 times line 64, column 4, lines 20 and 23. Column 8, line 30 before "measurement" insert a line 41, "measurment" should read measurement Column 9 line 30 "the" should read said Signed and Scaled this- A ttest:

RUTH C. MASON C. MARSHALL DANN (nmmissiumr ufParems and Trademarks .4 Nesting Officer 

1. A method for determining the moment of inertia of a rotating system without the necessity of appreciable perturbation of the operation of said system, wherein the inertial properties of said system and the losses and load acting on said system are constant or slowly varying, said method comprising; a measurement at a particular angular velocity of angular acceleration of a rotating element of said system with a certain non-zero torque acting on said system, using said measurement of angular acceleration to relate said moment of inertia to said torque, changing said moment of inertia either by adding to it or by subtracting from it a known moment of inertia, obtaining a second measure of angular acceleration of a rotating element of said system under equivalent conditions with only said moment of inertia changed by said known amount, using said second measure of angular acceleration to relate said changed moment of inertia to said torque, establishing said moment of inertia to be determined in terms of said known change in moment of inertia and said two values of angular acceleration by taking the difference between said two relations, averaging as many measurements as may be required to determine said moment of inertia to specified accuracy, and making said determined moment of inertia available for further use.
 2. A method for determining the moment of inertia of a rotating system without the necessity of appreciable perturbation of the operation of said system, wherein the inertial properties of said system and the losses and load acting on said system are constant or slowly varying, said method comprising; measurment at a particular angular velocity of angular acceleration of a rotating element of said system with a certain torque acting on said system, using said measurement of angular acceleration to relate said moment of inertia to said torque, changing said torque either by adding to it or by subtracting from it a known torque, obtaining a second measurement of angular acceleration of a rotating element of said system under equivalent conditions with only said torque changed by said known amount, using said second measurment to relate said moment of inertia to said changed torque, establishing said moment of inertia to be determined in terms of said known torque and said two values of angular acceleration by taking the difference between said two relations, averaging as many measurements as may be required to determine said moment of inertia to specified accuracy, and making said determined moment of inertia available for further use.
 3. A method for determining the moment of inertia of a rotating system at a specified value of measurable system parameters without the necessity of appreciable perturbation of the operation of said system, wherein the functional dependence of the torque acting on said system is known in terms of said system parameters, said method comprising; a simultaneous measurement by a controlled means at said specified value of said system parameters and with a certain non-zero torque acting on said system of angular acceleration of a rotating element of said system and of the particular value of said parameters, using said simultaneous measurement to relate angular acceleration to said moment of inertia and to said torque at said particular value of said parameters, aVeraging as many measurements as may be required to obtain said relation to specified accuracy at said specified value of said parameters, changing said moment of inertia either by adding to it or by subtracting from it a known moment of inertia, obtaining a second simultaneous measurement of angular acceleration of a rotating element of said system and of the particular value of said parameters under equivalent conditions with only said moment of inertia changed by said known amount, using said second measurement to relate angular acceleration to said changed moment of inertia and to said torque at said particular value of said parameters, averaging as many said second measurements as may be required to obtain second said relation to specified accuracy at said specified value of said parameters, establishing said moment of inertia to be determined in terms of said known change in moment of inertia and said two values of angular acceleration by taking the difference between said two relations at said specified value of system parameters, and making the result available for further use.
 4. A method for determining the moment of inertia of a rotating system at a specified value of measurable system parameters without the necessity of appreciable perturbation of the operation of said system, wherein the functional dependence of the torque acting on said system is known in terms of said system parameters, said method comprising; a simultaneous measurement by a controlled means at said specified value of said system parameters and with a certain torque acting on the system of angular acceleration of a rotating element of said system and of the particular value of said parameters, using said simultaneous measurement to relate angular acceleration to said moment of inertia and to said torque at said particular value of said parameters, averaging as many measurements as may be required to obtain said relation to specified accuracy at said specified value of said parameters, changing said torque either by adding to it or by subtracting from it a known torque, obtaining a second simultaneous measurement of angular acceleration of a rotating element of said system and of the particular value of said parameters under equivalent conditions with only said torque changed by said known amount, using said second measurement to relate angular acceleration to said moment of inertia and to said changed torque at said particular value of said parameters, averaging as many said second measurements as may be required to obtain second said relation to specified accuracy at said specified value of said parameters, establishing said moment of inertia to be determined in terms of said known torque and said two values of angular acceleration by taking the difference between said two relations at said specified value of system parameters, and making the result available for further use. 